1. FIELD OF THE INVENTION
The present invention relates to a metal vapor laser apparatus. It is particularly, but not exclusively, concerned with a copper vapor laser.
2. SUMMARY OF THE PRIOR ART
At its simplest, a metal vapor laser comprises a discharge tube with electrodes at each end. The introduced into the discharge tube. By applying a voltage between the electrodes, from a suitable power source, an electrical discharge can be set-up within the discharge tube, which vaporises the metal. The metal vapor is excited to emit laser light.
Normally, the discharge tube is made of an insulating heat resistant substance such as a ceramic. Therefore, it is necessary to provide a return current path from one of the electrodes to the power source, the power source normally being connected substantially to the other electrode.
One factor which is important in the design of a metal vapor laser is to provide means for dissipating heat generated during the discharge. For low power applications, air-cooling methods are normally used, in which the discharge tube is surrounded by a casing, and an air flow is introduced into the interior of the casing, to permit heat exchange between the discharge tube and that air, and so permit the heat to be removed from the discharge tube. In order to improve this effect, one or more vanes may be provided coaxially between the discharge tube and the casing. In such air-cooled arrangements, the casing is conductive, and it has been proposed in JP-A-62-26879 for that casing to provide a return current path from one of the electrodes to the power source.
Furthermore, in U.S. 4815091, it was proposed that the innermost vane between the casing and the discharge tube be connected between the power source and one electrode, to provide a second return path. It was believed that this would reduce the inductance of the discharge load, which inductance is primarily determined by the inductance in the return path(s).
For metal vapor lasers of high power, however, air cooling is not satisfactory, because it cannot dissipate heat sufficiently rapidly. Therefore, it is also known to provide a conductive jacket coaxially around the discharge tube, which jacket has inner and outer conductive walls with a space therebetween, so that the jacket forms a cooling vessel. If a liquid, such as oil, is introduced into that space, heat may be extracted thereby. Examples of metal vapor lasers using cooling jackets are disclosed in e.g. JU-A-63-102268 and the article entitled Large Scale Copper Vapor Laser Computer Control Systems, by M. J. LaChapell et al in ISA Annual Conference 1979.
When such a jacket is used, the jacket is normally located between a connection point to the power source and one of the electrodes, so that the conductive inner and outer walls of the jacket form two current return path.